Friction at a lubricated line contact operating at oscillating sliding velocities

Although many contacts operate under unsteady loading and sliding conditions, friction behavior under such conditions is still not well understood. In this paper we report on a series of experiments that were run to examine friction-velocity characteristics of line contacts operating under unsteady sliding velocities in the mixed, elastohydrodynamic and hydrodynamic lubrication regimes. A periodic, time-varying velocity component was superimposed on a steady sliding speed in such a way that all three lubrication regimes could be covered in a cycle. It was found that as the frequency of oscillation was increased, a multi-valved friction coefficient appeared as a loop about the average (steady state) friction-velocity relation. It is shown that this behavior can be modeled by a characteristic time lag between a changing velocity and the corresponding steady state friction.